Orthogonal functionalisation of upconverting NaYF4 nanocrystals

A simple and straightforward method for the orthogonal functionalisation of upconverting NaYF4 nanocrystals (UCNCs) - doped withYb3+ and Er3+ - based on N-(3-dimethylaminopropyl)-N′- ethylcarbodiimide/N-hydroxysuccinimide (EDC/NHS) selective reactions between two dyes and two different reactive groups present at the periphery of the upconverting nanocrystals is reported. Organic-soluble UCNCs of 10 and 50 nm in size are encapsulated efficiently in a 1:1 mixture of two commercial 3000 Da poly(ethylene glycol) derivatives with two different reactive groups (amino and carboxylic groups). The water-dispersible UCNCs are non-cytotoxic, stable in the physiological environment, and present free amine and carboxylic reactive groups on their periphery, allowing rapid, selective, and modular covalent conjugation to payloads through EDC/NHS reactions. PEG-encapsulated UCNCs with and without covalent conjugation to payloads are characterised in vitro through spectroscopic, dynamic light scattering, and electron microscopy measurements. Living cell analyses coupled with TEM measurements confirm the uptake and low cytotoxicity of the coated UCNCs. They are linked covalently to two different dyes, internalised by living cells, and analysed by confocal microscopy. The related colocalisation measurements prove the reactivity of both amines and carboxylic acids on the periphery of the nanocrystals. This approach demonstrates that it is possible to produce water-dispersible and cyto-compatible dual-functional UCNCs. Nanocrystals for living cells: Upconverting NaYF4 (Yb3+, Er3+) nanocrystals are usually synthesized in organic media. A simple post-synthetic process using modified PEGs has been developed and the resulting nanocrystals are water-soluble, bifunctional, and have low cytotoxicity. The possibility of linking two different dyes onto the same coated nanostructures through EDC/NHS chemistry is demonstrated, as well as their possible use in living cels for NIR biolabelling (see figure). © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.